The injection of water into hydrocarbon fields is a well-known technique in the oil and gas sector. It is an operation which consists in injecting water, commonly known as injection water, into an oil and gas well, on the one hand to recover the hydrocarbons and on the other hand to avoid the wells collapsing, which can come about due to the drop in pressure as a result of the hydrocarbons being extracted.
The origin of the injection water generally depends on its availability and on the constraints around the site of the hydrocarbon extraction. For example, in the case of offshore extraction, using water drawn from the sea is known. Treatment steps, however, are generally essential in order to obtain from the seawater water which has the quality sufficient to enable it to be reintroduced into the underground formation. The injection water is always obtained by means of a filtration step, aiming to reduce or even eliminate suspended matter, and by means of a deoxygenation step. Often a supplementary treatment in the form of desulphurization is used if the contents of the underground formation are such that reduction in the sulfate ions is necessary.
The injection water can also be aquifer water, river or lake water, and possibly domestic or industrial wastewater. Here too, treatment steps can be necessary to obtain water which has a quality which is compatible with injection into the underground formation.
When the injection water is seawater, the presence of sulfates in the water is typically a problem if the underground formation contains barium, calcium or strontium ions. Indeed, the sulfate ions form with the barium, calcium or strontium ions precipitates which create mineral deposits (scaling) and these are disadvantageous to good hydrocarbon extraction. Furthermore, the presence of sulphates can be the cause of the generation by bacteria of hydrogen sulfide (H2S), a toxic and corrosive gas, which can cause piping that is used for recovering hydrocarbons to corrode. The elimination of the sulfates from the water before it is injected into the underground formation is therefore often necessary.
A conventional method enabling the elimination of sulphates from the water consists in a nanofiltration membrane method which retains the multivalent ions and allows the monovalent ions to pass. Another conventional method enabling water desalination consists in a reverse osmosis method. Such methods are described, for example, in patent applications WO 2006/134367 and WO 2007/138327.
Generally, the water treatment units are placed close to the hydrocarbon field. In the case of underwater fields, said units are conventionally installed on the surface, on the offshore platform for extracting hydrocarbons or on attached floating platforms, currently called FPSO units (acronym of “Floating Production, Storage and Offloading” according to Anglo-Saxon terminology, signifying a floating unit for production, storage and offloading).
One of the major problems associated with installing water treatment units on the surface is the space required. Said units take up space. Yet the management of space on the offshore platforms is tricky as space is limited and many of the installations are essential. There is therefore a need for water treatment units which require minimum space on offshore platforms.
One solution to this problem has already been proposed in the prior art. It consists in replacing the existing treatment units by underwater units which are capable of operating underwater. In particular, international patent application WO 2009/122134 describes an underwater seawater treatment unit. The fact of putting the unit underwater also has an advantage in terms of power: the system placed in an underwater environment profits from hydrostatic pressure which is approximately proportional to the depth at which it is situated. U.S. Pat. No. 7,600,567 and patent application GB 2 451 008 also describe an underwater water treatment unit which is able to be placed underwater at a depth of between 250 and 700 meters inclusive.
Now currently, discoveries of large size hydrocarbon fields are made increasingly rarely in shallow and moderately deep waters, that is to say at depths of up to 500 meters. With the aim of ensuring renewal of reserves, it is necessary to develop new fields which are located at great depths (that is to say at a depth of between 500 and 1,500 meters inclusive) and at ultra-great depths (that is to say deeper than 1,500 meters).
In shallow and moderately deep waters, temperature and salinity conditions as well as aquatic fauna are not radically different to conditions on the surface. By contrast, at great depths and ultra-great depths, the water temperature is approximately between 3° C. and 5° C. and the water viscosity increases with the depth. Thus, the environmental conditions to which the underwater water treatment units are subjected are quite specific.
In addition, the true underwater environment cannot be defined solely by conditions of pressure and temperature. It is a complex environment, with variable chemical compositions, above all including micro-organisms that are specific to great depths and to the ultra-great depths.
Said micro-organisms tend to hang onto and build up on certain surfaces, and more particularly to clog up all underwater devices rapidly. Said phenomenon of clogging up which is biological in origin is currently designated by the Anglo-Saxon term of biofouling. It is a question of the degradation or deterioration of a surface or of an object left in an aquatic environment, as typically in the sea, by the growth of living organisms such as bacteria, protozoa, algae and crustaceans.
When they are placed at great depths or at ultra-great depths, the underwater water treatment units, and in particular the filtration membranes that they contain, are therefore subject to very specific environmental conditions. Traditionally, the operation of filtration membranes under said conditions of pressure, temperature and salinity has been reproduced in a laboratory.
However, said solution is not totally satisfactory. Indeed, at the present time, researchers do not have the knowledge to simulate in a reliable manner the phenomenon of biofouling in a laboratory as too many parameters, such as the nature of the bacteria, the sea currents, the seasonal changes, etc., cannot be reproduced.
Now, in the case of seawater treatment units with a view to using the water as injection water in an oil well, said units preferably have to remain operational for several months, or even several years without any intervention whatsoever. Furthermore, it is preferable for the unit to be as sturdy as possible so as to reduce the number of essential maintenance operations.
Thus, the study of the clogging up by biofouling of filters intended for the great depths and the ultra-great depths is particularly important as it would enable the maintenance devices and procedures that are the most suitable for underwater seawater treatment units to be defined with a view to the water being used as injection water in an oil well.
The problem posed is equally true for any underwater installation which has a filtration system. There are numerous cases where the researchers and manufacturers need to operate filters underwater, for example in the seas and oceans. For example, the devices for pumping seawater for analysis purposes or for industrial purposes, or the devices for research into and study of shipwrecks at great depths can be cited.
By way of example, patent application US 2011/0132842 describes a device and a method for purifying water, notably for human consumption, using a combination of a reverse osmosis unit and a dehumidification unit. Another example is given by patent application CN 101844002 which describes a filtration method and device which enable water samples to be taken and analyzed in-situ. However, none of said documents neither discloses nor even suggests a method for evaluating the clogging up of filters in-situ.
It is in said context that the inventors have developed a method for evaluating the operation of filters, which enables, using an underwater testing device, the clogging up of filters to be studied directly in the real underwater environment.